This might be a dumb question, but is it possible to make a .png file that isn't square?
If so, how can one do this in Gimp?
Ideally I'd like one in the shape of an X.
All image formats (even vector) must maintain a rectangular border specifying the size of the image. However, one can use the transparent properties of PNG images to simulate the appearance of a non-square image.
Just create your image/shape, ensure the background is transparent, and save the file in the PNG format. Make sure you include the alpha channel so there is a transparent background saved with the image.